1. Field of the Invention
The present invention relates generally to ocular surgical procedures involving implantable lenses, and more specifically to devices, systems and methods for the determination or selection of a lens power for providing emmetropic vision or, if chosen, a specific ametropic vision, taking into account various parameters of the eye.
2. Description of the Related Art
Intraocular Lenses (IOLs) may be used for restoring visual performance after a cataract or other ophthalmic procedure in which the natural crystalline lens is replaced with or supplemented by implantation of an IOL. Accurate determination of lens power is an important aspect in providing emmetropia, or a desired degree of ametropia. Measurements of the eye are typically made preoperatively and a lens power is selected based on correlations between the measured values and lens powers providing a desired refractive outcome.
Over the years a number of intraocular lens power calculation formulas have been developed, for example, as discussed in the book published by SLACK Incorporated entitled Intraocular Lens Power Calculations, by H. John Shammas. These power formulas may be broadly characterized into at least two categories: theoretical formulas, which are based on a geometric optic, two-lens vergence formula; and regression formulas, which are based on regression formulas obtained by fitting data from a large patient database to an equation relating lens power to one or more parameters thought to correlate with lens power. While progress has been made in the accuracy of intraocular lens power calculation formulas to obtain better refractive outcomes, undesirable refractive outcomes due to improper intraocular lens power calculations still occur. Apart from the general desire for spectacle-free refractive outcomes, demands for more accurate lens power calculation have also increased due to the introduction of multifocal, as well as accommodating IOLs.
Many of the current formula algorithms were derived by optical back-calculations to agree with a refractive outcome. In this manner they may be confounded with errors in all parameters used in the calculation, and the oversimplification of thin-lens theory. An evaluation of the sources of errors in lens power calculations was published by Sverker Norrby entitled “Sources of error in intraocular lens power calculation”, Journal of Cataract and Refractive Surgery, Vol. 34, pp. 368-376, March 2008. In this paper, preoperative estimation of postoperative intraocular lens position was determined to be the largest contributor of error in the refractive outcome of cataract surgery, with an error contribution of 35%, relative to all error sources evaluated. Another publication by Olson (“Calculation of intraocular lens power: a review.” Acta Opthalmologica Scandinavica 2007; 85:472-485) reports the same order of magnitude for the same source of error.
In most, if not all of the current formula algorithms, there are a number of ocular parameters that are used in deriving an appropriate lens power for implantation into the eye. These parameters include axial length (AL), corneal radius (CR) or power (K), and anterior chamber depth prior to surgery (ACDpre), among others. In general, one or more of these parameters are used to provide the preoperative estimation of the postoperative effective lens position (ELP), which is related to the IOL's principal plane, although it may be modified depending on the surgeon through the optimization of the corresponding IOL constant. The ELP is then used in combination with one or more of these same parameters to provide an estimate of the correct lens power to provide a desired refractive outcome (typically emmetropia).
For example, in the SRK/T method, the empirical calculation based on regressions is used to predict the ELP in the eye after surgery. Once that position is known, the IOL power to implant is calculated by simple paraxial optics, taking into account that the eye is a two lens system (cornea+IOL), focusing on the retina. This approach is based on Fyodorov's theoretical formula. However, as discussed above, calculating ELP is a large error source in this process. Accordingly, better systems and methods are needed that will allow reliable and accurate determination of an implanted lens' power.